As is well known in the art, when a resilient material plunger is inserted into a closed chamber, the movement of the plunger from the open end of the chamber into the chamber compresses the air or gas that is ahead of the plunger. The greater the amount of movement the greater the amount of pressure build up. As a result this insertion pressure, plus additional pressure due to temperature buildup, plus any reduction in pressure behind the piston will cause the piston to move back out from the chamber. Many techniques are used to prevent this from occurring. These include use of a bleed wire alongside the piston which allows the pressurized gas or air to escape alongside the bleed wire, inserting the piston through a tube which fits inside the opening of the chamber thus allowing the pressurized gas or air to flow past the tube since the tube is not a sealing member, or the plunger can be inserted into the chamber after the chamber has been evacuated in a vacuum chamber. Although all of these techniques work, they are expensive, they slow down production, and in sterile operations these techniques introduce other factors which can easily effect sterility. In my patent U.S. Pat. No. 4,929,230 is disclosed a plunger design which under certain conditions allows material to flow past the plunger. This invention changes the design of the plunger of the above mentioned patent to achieve the desired results of having the plunger bypass gas or air pressure under one condition of operation and still is capable of providing pressure on the contents of the chamber or provide pressure on the contents to eject the contents from the chamber.